The present invention relates to a precision electronic balance made up essentially of a servo system containing therein an electromagnetic force transducer which produces a force to counterbalance weight of a body or bodies to be weighed.
In this type of electronic balance, the electromagnetic force transducer is made operative either with a stationary DC current or with a pulse current, and the weight is determined by a "balancing current" being supplied to the force transducer when the servo system is brought into and kept at an equilibrium state. If the balancing current is a stationary DC current, its magnitude corresponds to the weight, while, if the balancing current is a pulse current, its duty factor or pulse width gives the weight.
With a stationary DC current used, precision measurement of the current necessiates a high-resolving A-D conversion system, because the measurement is made through a process of digitalizing the magnitude of the current. The use of a high-resolving A-D conversion system not only increases the cost of the balance but also generally makes it take a relatively long time to complete one operation of weight measurement. Further, the whole servo system is necessarily kept very stable for that long time needed for the measurement.
In the case where the force transducer is operated with a pulse current, its duty factor is varied to counterbalance the weight, and the weight is determined by measuring the pulse width giving the duty factor. The measurement is made by counting the number of clock pulses contained in the time interval equal to the pulse width. In this case, the pulse current must have its period kept at 2 milliseconds or shorter to avoid a possible vibrational motion of the balance. Therefore, with a 30-MHz clock used, the number of clock pulses countable within the pulse width is restricted to 60,000 (16 bits) at the largest, failing to give a precision highly resolved value of weight. This maximum countable number also corresponds to the ability of commercially avilable counter ICs. Although, for instance, Japanese Laid-open Patent Application No. 54-48277 proposes means for overcoming the drawback caused by the above limit imposed on the clock pulse counting, the technology disclosed therein contains a process of integration and makes the constitution of a balance disadvantageously complicated.
There is another improved method in which are provided both an additional servo system for fine adjustment and a plurality of pulse current sources for generating various pulse currents discontinuously different in width. Weight is first counterbalanced coarsely with one of the pulse currents, and then a residual value of the weight is finely offset through the additional servo system. This method also has a disadvantage that, when the weight loaded on the balance changes continually or suddenly as is experienced, for instance, in the process of weighing out some items, the balance often overswings owing to a possible temporary pulse-current misslection due to the response characteristics of PID control circuits employed in the servo system, causing a weight value display to wide fluctuate.